1. Field of the Invention
The present invention relates to an active matrix display device, and more particularly to technology for taking as input and displaying a picture for an image signal obeying a wide standard having, for example, a 16:9 aspect ratio on an active matrix display device having a screen which obeys, for example, a normal standard having a 4:3 aspect ratio.
2. Description of Related Art
In recent years, the range of applications for active matrix display device employing liquid crystal display panels has increased in scope so as to include, for example, camera view-finders. Recent video cameras are capable of selectively designating aspect ratios for photographing which obey a normal standard (for example, an NTSC standard or PAL standard etc.) and aspect ratios for photographing which obey a wide standard (for example, a HDTV standard, etc.). However, active matrix display device having an aspect ratio obeying the usual normal standard are used as viewfinders.
If an image photographed at a wide standard aspect ratio is projected onto an active matrix display device having a normal standard aspect ratio, the picture is transformed so as to be extended in the vertical direction because the longitudinal dimensions of the picture are relatively compressed compared with the vertical dimensions. It is preferable for the photographed wide image to be displayed without modification even, for example, in the case of a video camera viewfinder. However, as the active matrix display device using the monitor has a usual normal standard aspect ratio, projection is not possible without modifying the picture.
When displaying a wide standard image as a normal standard picture, it is necessary to compress the image in the longitudinal direction. In this case, remaining portions occur at the top and bottom of the normal standard picture. In order that the entire picture is easy to see, it is preferable to show a dark level background at the remaining portions. A normal standard active matrix display device has 225 lines, in, for example, the case of a half-line structure. It is necessary to compress this to 169 horizontal lines in accordance with the aspect ratio dimensions for the case where a wide standard image is displayed. As an image signal for 169 horizontal lines is to be written in the time allotted to 225 lines, the time remaining to write a dark signal to the remaining portions is comparatively short. In this short time, 28.times.2=56 horizontal lines for the dark signal have to be written to the top and bottom. For example, as 225 horizontal lines are allotted in a half-line structure for the case of an NTSC standard where there are 262.5 scanning lines per field, a time corresponding to 37.5 horizontal scanning lines is provided as an overscanning period. In the case where the dark signal is written utilizing this overscanning period, a dark signal corresponding to 56 horizontal lines has to be written at the top and bottom in a period of time corresponding to 37.5 horizontal scanning lines. This period of time is therefore completely insufficient.